WILLY T tells you how to squash sound into manageable shapes with COMPRESSORS & LIMITERS.

Many of the sounds we hear at a kick-arse rock gig have a huge difference between the loudest and softest parts of songs. The singer can go straight from a whisper to a roar in milli-seconds. Compressors are used to ‘smooth-out’ these wild variations by raising the level of quiet passages and/or lowering the levels of the loud passages. Technically they reduce the Dynamic Range of the programme material.

Basically, compressors do what we do when we are ‘riding the faders’ at our mixing desk to keep the various levels of the various sounds under control. For example we automatically pull down the lead singer’s levels when they are really going for it, and bring them up during the quieter, more delicate moments.

Compressors do what our brains and ears are telling us to do, but they react much, much faster, with a greater degree of accuracy and precision than even the best soundie could ever hope for. Plus compressors don’t get distracted by saucy punters or ‘tired & emotional’ at the gig.

Compressors reaction to, and control of, excessive changes in levels is so fast as to be practically instantaneous.

Both rack-mounted hardware and software compressor plug-ins share similar controls as they are all doing the same job and need the same control functions to work properly.

The Threshold is the adjustable level which tells the compressor when to jump-in and do-its-stuff (calibrated in decibels – dBs). If the signal is below the threshold set by the user, nothing happens to it, but as soon as it crosses the threshold, it is turned down by the specific amount.

Compressors without an adjustable threshold level usually have an Input or Sensitivity Level, with an internally fixed Threshold Point. We can adjust the input level closer to, or further away from the fixed threshold, kicking the compressor into action. Too easy.

In some units the compressors’ action can be set-up to act immediately as the threshold is crossed in what’s known as a Hard-knee setting, or, more gradually with a Soft-knee setting.

The Ratio knob decides how harsh the gain reduction is, once the signal crosses the threshold. It is a ratio of Input verses Output levels. The higher the ratio, the more gain reduction is applied and the stronger the compression effect.

Expressed in dBs, ratio is defined as the number of dB by which the input level needs to increase to cause a corresponding 1dB rise in output level. For example, if a compression ratio of 3:1 is set, an input signal exceeding the Threshold by 6dB will only cause a 2dB increase in level at the output.

The Attack controls the time it takes the compressor to react and initiate gain reduction after the signal has reached or crossed the threshold. A fast attack will bring the signal under control immediately, which is effective for limiting. A slower attack will allow the start of a transient or percussive sound – eg. the crack of a snare drum or the slap of a bass guitar – to pass through uncompressed, emphasising the sharp hit of the transients of percussion sounds. A slow attack on bass sounds will avoid ‘clicking’ due to the compressor opening mid-cycle of the low frequency waveform.

The Release control sets the amount of time it takes for the compressor’s gain to return to normal once the signal has fallen below the threshold. Release times should be long enough to track the sound envelope smoothly. If the Release Time is set too short, there will be the audible effect of ‘pumping and breathing’ as the background noise level rises and falls in time with the action of the compressor. To avoid pumping and breathing in typical music applications, a good starting value for the Release Time is 0.5 seconds. If the Release Time is set too long, the quieter parts of the song or music may well be still be compressed in response to higher-level signals above the threshold.

So that’s a compressor, what’s a limiter? A limiter is just a compressor with a very high ratio – 10:1 or more – and a threshold set to close to the maximum allowable peak signal level. If the ratio is made high enough, the signal is prevented from ever getting past the threshold and there will be no change in output level no matter how much the input level changes. This is known as limiting, because the threshold sets a limit that the output is never allowed to exceed. Technically, a limiter should have a theoretical compression ratio of Infinity:1. But in the real-world, any ratio above around 10:1 is close enough to be effectively limiting the signal. Brick Wall Limiters – with an ∞:1 ratio – are commonly used for Broadcast or Digital Equipment, where the signal must never, ever cross a set level, or massive distortion takes place.

Compression can be carried out in a swag of different ways, with different electronic formats inside – Analogue, Digital, Optical. They can be hardware based or be ‘virtual’ as software plug-ins running on computer. Therefore they all sound different, no matter now "transparent" the manufacturer says they are! These differences give us space to work our engineer’s magic on the sounds. Viva la difference I say!

WILLY T operates Giraffe Media, supplying Words, Pix & Live Recording Services to the Entertainment Industry. Email .

Be first to comment on this article

Write Comment
Comments are submitted for possible publication on the condition that they may be edited. Poster's IP addresses are logged.
Name:
Comment:
Code:*